[opt-viewer] Don't Decode HTML bytes for Python 2

[opt-viewer] Don't Decode HTML bytes for Python 2

Summary:
D36624 added some python3 compatibility. But that fix has a problem..

With python2 (which is specified by `#!/usr/bin/env python2.7`), if the env variables do not specify the UTF8,
and the source file is UTF8 (contains non-ASCII symbols), then the `.decode('utf-8')` causes the following exception:
```
Reading YAML files...
Rendering HTML files...
        8 of 41Traceback (most recent call last):
  File "/build/llvm/tools/opt-viewer/opt-viewer.py", line 277, in <module>
    print_progress)
  File "/build/llvm/tools/opt-viewer/opt-viewer.py", line 213, in generate_report
    should_print_progress)
  File "/build/llvm/tools/opt-viewer/optpmap.py", line 45, in pmap
    result = map(_wrapped_func, func_and_args, *args, **kwargs)
  File "/build/llvm/tools/opt-viewer/optpmap.py", line 25, in _wrapped_func
    return func(argument)
  File "/build/llvm/tools/opt-viewer/opt-viewer.py", line 174, in _render_file
    SourceFileRenderer(source_dir, output_dir, filename).render(remarks)
  File "/build/llvm/tools/opt-viewer/opt-viewer.py", line 125, in render
    self.render_source_lines(self.source_stream, line_remarks)
  File "/build/llvm/tools/opt-viewer/opt-viewer.py", line 79, in render_source_lines
    </tr>'''.format(**locals()), file=self.stream)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xf4' in position 47: ordinal not in range(128)
```
This is similar to https://bugs.llvm.org/show_bug.cgi?id=33548, which was fixed by https://reviews.llvm.org/D37661

Unlike that fix, here, *removing* `.decode('utf-8')` actually fixes it.

Since i assume that the original fix is needed, i simply made
that fix conditional, since for python2 it actually breaks things.

Reviewers: modocache, anemet

Reviewed By: anemet

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D38289

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315350 91177308-0d34-0410-b5e6-96231b3b80d8